Some respiratory masks are categorized as “disposable” because they are intended to be used for relatively short time periods. These masks are typically made from nonwoven fibrous webs and generally fall into one of two categories, namely, fold-flat masks and shaped masks. Fold-flat masks are packed flat but are formed with seams, pleats, and/or folds that enable them to be opened into a cup-shaped configuration. In contrast, shaped masks are more-or-less permanently formed into a desired face-fitting configuration and generally retain that configuration during use.
Shaped masks regularly include a supporting structure, generally referred to as a “shaping layer”, that is commonly made from thermally bonding fibers, which are fibers that bond to adjacent fibers upon being heated and cooled. Examples of face masks that are formed from such fibers are disclosed in U.S. Pat. No. 4,807,619 to Dyrud and U.S. Pat. No. 4,536,440 to Berg. The face masks that are disclosed in these patents comprise a cup-shaped mask body that has at least one shaping layer (sometimes referred to as a “shape retaining layer” or “shell”) that supports a filtration layer. Relative to the filtration layer, the shaping layer may reside on an inner portion of the mask (adjacent to the face of the wearer), or it may reside on an outer portion of the mask or on both inner and outer portions. Typically, the filtration layer resides outside the inner shaping layer. Shaping layers also may be made from other materials such as a network or mesh of plastic strands—see, for example, U.S. Pat. No. 4,850,347 to Skov.
In making a mask body for a molded filtering face mask, the filtration layer is typically juxtaposed against at least one shaping layer, and the assembled layers are subjected to a molding operation by, for example, placing the assembled layers between heated male and female mold parts—see U.S. Pat. No. 4,536,440 to Berg. Alternatively, a molded mask body has been made by (1) passing a layer of filtering material and a layer of thermally-bondable fibers together in superimposed relation through a heating stage where the thermally bonded fibers, or at least one component of the fibers is softened, and thereafter (2) molding the superimposed layers to the shape of a face mask in molding members that are a temperature below the softening temperature of the thermally-bonding fibers—see U.S. Pat. No. 5,307,796 to Kronzer et al.
In known commercially available products, the filtration layer, whether made by either of the above-noted techniques, typically becomes attached to the shaping layer by entanglement of the fibers at the interface between the layers and usually also by some binding of the fibers of the shaping layer to the filtration layer—see U.S. Pat. No. 4,807,619 to Dyrud et al. In addition, known masks commonly have a seal about the periphery of the mask body to join the assembled layers together. Although commercially available masks commonly join the filtration layer to the shaping layer as just described, U.S. Pat. No. 6,041,782 to Angadjivand et al. indicates that the filter layer may be bonded to the shaping layer shell across its entire inner surface through use of, for example, an appropriate adhesive.
Although the art recognizes a variety of ways to manufacture molded filtering face masks, it has nonetheless left room for improvement in the construction of such a product. After being worn numerous times and being subjected to high quantities of moisture from a wearer's exhalations, in conjunction with having the mask bump into other objects while being worn on a person's face, known masks can be susceptible to collapsing or having an indentation pressed into the shell. The wearer can remove this indentation by displacing the mask from the face and pressing on the indentation from the mask interior.